1. Field of the Invention
The present invention relates generally to a measurement device for measuring a variable, e.g., temperature difference, with an automatic offset error compensation, and more specifically to a measurement device which automatically compensates for discrepancies in operating characteristics due to aging and/or random effects of dispersed circuit parameters and variations during manufacture in an amplifying section which amplifies a detector voltage signal according to changes in the variable to be measured.
2. Description of the Prior Art
In the case where a variable such as displacement, pressure, flow rate, and temperature, etc., (hereinafter referred simply to as a parameter) is detected in the form of a voltage, the detected voltage is amplified by the measurement device and the amplified voltage is processed by a subsequent calculating circuit so as to obtain a measurement value.
An automotive vehicle engine requires a measurement of the temperature difference (.DELTA.T=T.sub.2 -T.sub.1) between the temperature (T.sub.1) of intake air from outside of the engine and the temperature (T.sub.2) of the intake air intermixed with exhaust gas.
The temperature difference described above can be measured by means of a thermocouple utilizing the Seebeck effect.
If two junctions of two different kinds of metals are disposed at higher and lower temperature sections, respectively, a thermoelectromotive force is generated according to the temperature difference .DELTA.T and consequently a voltage (V.sub.2 -V.sub.1) is produced according to the detected temperature difference and is outputted to an amplifying circuit. The amplified voltage is processed by a calculating circuit to convert a desired temperature difference, as is shown in FIG. 1.
The amplifying circuit described above comprises the following items, as shown in FIG. 2:
(a) a first operational amplifier having a noninverting input terminal connected to one terminal of the thermocouple to receive a lower voltage V.sub.1 indicative of the thermo-electromotive force generated by the thermocouple on the lower temperature side and connected to a constant voltage supply V.sub.0 via first resistor Ro, an inverting input terminal connected to the constant voltage supply V.sub.0 via a second resistor R.sub.1, and an output terminal thereof connected to the inverting input terminal via a third resistor R.sub.2 ; (b) a second operational amplifier having a noninverting input terminal connected to the other terminal of the thermocouple to receive a higher voltage V.sub.2 indicative of the thermo-electromotive force generated by the thermocouple on the higher temperature side, an inverting input terminal connected to the output terminal of the first operational amplifier via a fourth resistor R.sub.3, and an output terminal thereof connected to the inverting input terminal of the second operational amplifier via a fifth resistor R.sub.4 ; and (c) a third operational amplifier having a noninverting input terminal connected to the output terminal of the second operational amplifier, an inverting input terminal connected to ground via a sixth resistor R.sub.5, and an output terminal connected to its inverting input terminal via a seventh resistor R.sub.6. In the processing circuit described above, if the ratio of the resistances of the resistors are expressed as .alpha.=(R1/R2)=(R4/R3), .beta.=(R6/R5), and w.sub.1, w.sub.2, and w.sub.3 are respective offset voltages of the first, second, and third operational amplifiers, the output voltage u.sub.1 of the first operational amplifier and output voltage u.sub.2 of the second operational amplifier can be expressed, respectively, in the following equations: EQU u.sub.1 =(1/.alpha.)(V.sub.1 -V.sub.0 +w.sub.1)+V.sub.1 +w.sub.1 ( 1) EQU u.sub.2 =.alpha.(V.sub.2 -u.sub.1 +w.sub.2)+V.sub.2 +w.sub.2 ( 2)
in addition, the output voltage E of the processing circuit can be expressed by the equation: EQU E=(1+.beta.)(u.sub.2 +w.sub.3) (3)
From these three equations (1), (2), and (3), the output voltage E can be rewritten as: EQU E=(1+.alpha.)(1+.beta.)(V.sub.2 -V.sub.1)+(1+.beta.){(1+.alpha.)(w.sub.2 -w.sub.1)+w.sub.3 +V.sub.0 } (4)
, wherein (1+.alpha.) and (1+.beta.) are amplification factors of the associated operational amplifiers.
In the equation (4) expressed above, the first item on the right side of the equation indicates a pure amplified voltage corresponding to the thermoelectromotive force on the basis of the difference in temperature and the second item on the right side of the equation represents a voltage including the offset voltage of each of the operational amplifiers. It should be noted that because of irregularities in thermo-electromotive-force characteristics of each thermocouple, the first item inherently includes another offset voltage factor such that the difference between the higher and lower thermoelectromotive forces (V.sub.2 -V.sub.1) will probably not be zero when the ambient temperature is at 0.degree. C., i.e., the temperatures at both junctions is 0.degree. C.
Hoever, there are drawbacks in such a conventional measurement device. When many such measurement devices are massproduced, the circuit parameters of the resistors R.sub.0 through R.sub.6 and constant voltage V.sub.0 all need to be adjusted to make the amplification factor (1+.beta.)(1+.alpha.) and the offset voltage (1+.beta.){(1+.alpha.)(w.sub.2 -w.sub.1)+w.sub.3 +V.sub.0 } of each amplifying circuit equal to those of other measurement devices. This adjustment requires a great amount of labor in manufacturing of the measurement devices. Furthermore, after the circuit parameters and offset voltage of each measurement device are adjusted, the characteristics of the thermocouple used in the measurement device may change due to heat when the thermocouple is soldered into the measurement device and the operating characteristics of the measurement device itself may change due to the effects of aging.
Therefore, errors may occur in the output voltage E of the amplifying circuit and measurement errors may occur in the output value of the calculating circuit of the measurement device.